Thermionic hollow cathodes are traditionally used to ignite plasma propulsion thrusters such as Hall thrusters and gridded ion thrusters for spacecraft propulsion, and the cathodes can also neutralize the positively charged ion beam with negatively charged electrons and serve as the negative electrode of the thrusters' electric circuit. The cathodes function by heating a specially chosen material, the thermionic electron emitter, to temperatures sufficient to essentially “boil” off electrons into free space. Most materials melt before this occurs to any significant degree, so in practice only two materials are currently used, both of which operate typically in excess of 1,000° C. and up as high as 1,700° C.
Maintaining these high temperatures (T) requires large power inputs, primarily due to radiation losses proportional to the fourth power of temperature (T4) and conduction losses through the cathode mechanical structure. As a result, substantial effort is put into minimizing both effects in the cathode design. As shown in FIGS. 1 and 3, the cathode body 100 or 300 is typically constructed as a long tube with thin walls 102 or 302, with the emitter at the tip 108 or 308, to minimize conductive losses, while the tip is also wrapped in several layers of a refractory metal foil 114 or 314 such as tantalum to reduce radiation losses.
However, these long, thin tubes 102 or 302 are difficult to fabricate, and the radiation shielding 114 or 314 can present an electrical shorting risk in the cathode. The long tubes 102 or 302 also have to survive launch loads to get into space, including vibration and shock, thus requiring a certain amount of mechanical strength. Both of these constraints effectively limit both the aspect ratio (length/diameter) of the tube 102 or 302, longer tubes with small diameter being harder to fabricate and qualify, and also the wall thickness that can be achieved via conventional machining techniques, thinner walls being more difficult. These limits hamper efforts to reduce conductive losses by increasing thermal resistance via longer and thinner conductive pathways. Additionally, radiation shielding 114 or 314 reaches a point of diminishing returns due to conduction from layer to layer across their continuous wrapping, and the shielding 114 or 314 prevents only radiation in the radial direction when applied as a tubular layer of foil, which makes it difficult to prevent axial radiation losses away from the hot emitting tip area in either the upstream or downstream directions parallel to the cathode central axis.